Increased regulation of emissions from gas turbine engines has prompted the development of thermally efficient and reduced emission fuel injectors. It is known that carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions can be reduced with high temperature combustion in the engine. However, high temperature combustion can result in increased production of nitrogen oxides (NOX). This problem has been addressed by injectors which are configured more thoroughly to mix fuel with air prior to combustion with a fuel-to-air ratio below the stoichiometric level. Such arrangements can provide a “lean burn” which results in lower flame temperatures than would occur with stoichiometric burning. Since the production of NOX is a strong function of temperature, a reduced flame temperature or “lean burn” results in lower levels of NOX.
Staged fuel injection is known to result in reduced engine emissions. In such arrangements, the combustion process is divided into two (or more) zones or stages. These stages are typically physically separate from each other, but close enough to permit interaction. Separation can be axial and/or radial separation. A first, pilot stage is configured to provide fuel for low power operations. In higher power conditions, the pilot continues to provide fuel to the engine and in addition a second, main stage provides the remaining fuel needed for engine operation. In this way, proper fuel-to-air ratios can be controlled for efficient combustion, reduced emissions, and good stability.
Along with staged combustion, pollutant emissions can be reduced by providing a more thoroughly mixed fuel-air mixture prior to combustion wherein the fuel-to-air ratio is below the stoichiometric level so that the combustion occurs at lean conditions. Lean burning results in lower flame temperatures than would occur with stoichiometric burning. Since the production of NOX is a strong function of temperature, a reduced flame temperature results in lower levels of NOX. The technology of directly injecting liquid fuel into the combustion chamber and enabling rapid mixing with air at lean fuel-to-air ratios is called lean direct injection (LDI).
With an ever increasing demand for higher temperature operability, the heat load on some components of the injector becomes less tolerable and the operational life of the injector limited.
There is a desire to increase the operational life of staged fuel injectors.